Active Matrix/Organic Light-Emitting Displays (AMOLEDs) are one of hotspots in the research field of flat panel display today. Compared with Liquid Crystal Displays (LCDs), Organic Light-Emitting Diodes (OLEDs) have advantages such as low energy consumption, a low production cost, self-illumination, a wide angle of view, a fast response speed or the like. Currently, in the display field of mobile phones, PDAs, digital cameras or the like, OLEDs have begun to replace conventional LCD screens.
Compared with Thin Film Field Effect Transistor (TFT)-LCDs using a stable voltage to control brightness, OLEDs belong to current drive, and need stable current to control light emitting. As shown in FIG. 1, a conventional AMOLED pixel driving circuit is implemented using a 2T1C pixel driving circuit. The circuit only comprises one Driving Thin Film Transistor (DTFT), one switch Thin Film Transistor (TFT) (i.e., T1) and one storage capacitor C. When a certain row is gated (i.e., scanned) by scanning lines, a scanning signal Vscan is at a low level, T1 is turned on, and a data signal Vdata is written into the storage capacitor C. After the scanning for this row ends, Vscan is converted into a high level signal, T1 is turned off, and the DTFT is driven by a gate voltage stored in the storage capacitor C, to generate current to drive the OLED, so as to ensure that the OLED continuously emits light in one frame of display. A current equation when the driving thin film transistor DTFT reaches saturation is Ioled=K(Vgs−Vth)^2, wherein K is a parameter related to a process and a design, Vgs is a gate-source voltage for driving the thin film transistor, and Vth is a threshold voltage for driving the thin film transistor. Once the size and process of the transistor are determined, the parameter K is determined. FIG. 2 illustrates a timing diagram of an operation of the pixel driving circuit illustrated in FIG. 1, i.e., illustrating a timing relationship between a scanning signal provided by the scanning lines and a data signal provided by data line.
The AMOLED can emit light since it is driven by current generated by the driving thin film transistor DTFT in a saturation state. No matter a Low Temperature Poly Silicon (LTPS) process or an Oxide process is used, due to non-uniformity of the processes, threshold voltages of the driving thin film transistor DTFT in different positions may differ. Since when the same driving voltage is input, different threshold voltages may cause generation of different driving currents, inconsistency of current flowing through the OLED may occur, which results in non-uniformity of display brightness, thereby influencing the display effect of the whole image.
The existing proposed solutions are to add a compensation unit in each pixel to eliminate the influence of the threshold voltage Vth by compensating for the driving transistor. However, most of the existing AMOLED compensation units require a data write switch to turn on all the time in the threshold voltage compensation phase of the driving transistor, until the driving transistor is turned off automatically. This phase lasts for a long time. For a high-resolution AMOLED panel, data write time for each row of pixels becomes increasingly short. However, for a circuit which requires the data write switch to turn on all the time in the compensation phase, the threshold voltage cannot be acquired in short write time, and thereby the circuit cannot support the high-resolution AMOLED panel.
Therefore, there is a need for a pixel driving circuit and method which can shorten write time of a data voltage while ensuring that there is enough time to compensate for the threshold voltage of the driving unit.